The present invention relates to a printer, and more particularly, to a method of controlling printing positions in a serial printer of a type adapted for bi-directional printing, which method is capable of preventing or reducing dislocation of printing positions, and an apparatus therefor.
Generally, a serial printer comprises a home position sensor for printing position control, which is arranged to generate an output signal when a carriage driven by a motor reaches one end of a printing area. In synchronism with the fall of this output signal occurring upon the carriage reaching its reference moving position, an operation for counting dot pulses is started so as to control printing positions on the basis of the count, each dot pulse being generated each time the carriage driving motor rotates for a predetermined rotational angle. Ordinarily, the counting operation is carried out in synchronism with generation of a detection edge of the dot pulse, which is comprised of either one of the leading (rising) and trailing (falling) edges of the pulse.
However, the generation timing of the detection edge of the dot pulse can vary due to vibration of a drive belt or a pulley shaft of a carriage driving mechanism, for instance. In this respect, in case that the trailing edge of the output signal supplied from the home position sensor, which may be used as a trigger signal for causing the dot pulse counting operation to start, is generated in a closed timing relation with the generation of the detection edge of the dot pulse, the detection edge of the dot pulse to be counted can be generated just before the fall of the sensor output despite the requirement that such detection edge should be generated after the fall of the sensor output. As a result of this kind of fluctuation in generation timing of the detection edge with respect to the fall of the sensor output, the detection edge of the dot pulse to be counted is counted or is not counted, resulting in an uncontrollable dislocation of printing positions.
For instance, let it be assumed that a regular printing section of a serial printer of a type adapted for reciprocal or bi-directional printing starts at a point of P1 and ends at a point Q1 (see FIG. 6(c)), the count i of dot pulses, indicative of the printing start position in the forward movement of the carriage, and the count m, indicative of the printing end position in the backward movement, are set to values corresponding to the point of P1, respectively. Further, the count i, indicative of the end position in the forward movement, and the count k, indicative of the start position in the backward movement, are set to values corresponding to the point of Q1, respectively (see FIG. 6(a)). If the dot pulse to be counted immediately after generation of the trailing edge of the home position sensor output is not counted due to an uncontrollable shift of generation timing thereof, for instance, the respective counts of dot pulses at the position of P1 in the forward and backward movements of the carriage equal to i-1 and m-1; and the respective counts at the position of Q1 equal to j-1 and k-1 (see FIG. 6(b)). As a result, the printing start position in the forward movement of the carriage in accordance with the printing position control parameter i is dislocated from the desired position P1 to the right by one dot pulse, to be equal to a position P2, whereas the printing end position in the backward movement in accordance with the parameter m is dislocated to the left by one dot pulse, to be equal to a position P3. Thus, a dislocation corresponding to two dot pulses appears between the printing positions P2 and P3 (see FIG. 6(d)). Further, the printing end position in the forward movement in accordance with the parameter i is dislocated to the right by one dot pulse, to be equal to a position Q2, and the printing start position in the backward movement in accordance with the parameter k is dislocated to the left by one dot pulse, to be equal to a position Q3, with a dislocation of two dot pulses found between the printing positions Q2 and Q3 (see FIG. 6(d)).
In this manner, if the dot pulse to be counted is not counted due to the presence of a closed generation timing relation between the trailing edge of the home position sensor output and the detection edge of the dot pulse, a dislocation corresponding to two dot pulses occurs between adjacent printing lines respectively associated with the forward and backward movements. Such misalignment of the adjacent printing lines resulting from the above-mentioned dislocation makes the resultant printed text poor in quality.
Conventionally, to obviate this, fine adjustment of mounting position of the home position sensor is made upon assemblage of the printer. However, such adjustment requires labor. Further, the mounting position of the home position sensor varies with a change in temperature around the printer and due to the presence of deterioration with age and wear of associated parts of the printer. Thus it is difficult to prevent dislocation of printing position for a long service life of the printer.